In the prior art nuclear magnetic resonance analysis has, in particular instances been directed at composite samples for concurrent measurement. For example, it has been known to employ multiple discrete samples to construct phantoms for NMR imaging studies. In one typical example such a phantom comprises several sample tubes arranged with axes parallel with and subject to an inhomogeneous magnetic field of controlled gradient. Spatial discrimination is the object of such apparatus and the output derived is proportional to spatial coordinates describing the density distribution of the discrete samples (of identical composition) forming the phantom. All samples in such instance are of a single common substance for which the resonance frequency varies in accord with the field gradient.
It is also known in the prior art to extract data from an observe channel and a lock channel of an NMR spectrometer. In one class of experiment a calibration substance is intermixed in the sample ("internal reference") as described in U.S. Pat. Nos. 3,427,532 and 3,530,371. The samples are not "discrete" because it is desired to obtain field sampling distributed over the same volume as occupied by the sample of interest. In another class of lock channel apparatus the signal from a calibration substance is separately analyzed ("external reference") to produce a field-frequency control signal for maintaining a constant ratio of polarizing field to excitation frequency. Examples of this type of lock channel apparatus are U.S. Pat. Nos. 3,495,162 and 3,434,043. For both internal and external references, it is important to recognize that this known substance is a reference and not subject to "analysis." Rather, the reference provides a signal for actuating a control loop. In the context of lock channel systems, a reference provides a decidedly known spectral feature, the position of which is fixed by definition for the known lock system, and the spectral response of the instrument for the analyte is affected by the lock channel. Thus, the reference and the sample spectral responses are not each free to vary independently.
It is also known in prior art to confine a sample in an axial direction within a sample tube by use of plugs selected by the magnetic susceptibility of the plug material. The volume magnetic susceptibility of the plug is matched to that of the gross volume magnetic susceptibility of the sample solution such that no axial discontinuity is introduced by the plug. In this manner the axial extent of the sample may be defined within a selected portion of the sensitive volume of the probe coil of a magnetic spectrometer. This technique is described in U.S. Ser. No. 482,344, U.S. Pat. No. 4,549,137.
For the present invention it is assumed that a large sample volume does not substantially improve the quality of the spectrometer analysis for the purposes of the investigation. The present apparatus, in one embodiment, is designed to improve throughput for a plurality of samples to be analyzed.
Throughput is improved in the present invention by performing analyses of a plurality of samples concurrently. It is recognized that many subsystems can be effectively shared among a number of samples. For example, the polarizing field and the RF excitation apparatus are capable of operating upon several samples concurrently present in the sensitive volume of the spectrometer. As discussed herein, the signal processing apparatus may be partitioned to separately address commonly excited samples, or where the samples are suitably disparate in spectral response, a single RF signal processor apparatus is adequate to process the superposition of signals.
In another embodiment, concurrent analysis of disparate samples is exploited where it is again desired to improve throughput obtaining superimposed spectra, the features of which are adequately attributable to respective samples. In a slightly different operative context, one said disparate sample serves functionally as an analytic, or spectral, reference (as opposed to an instrumental reference in lock systems discussed above). The spectral amplitude or position of a reference feature is added for comparative purposes to the composite spectrum.